1. Field of the Invention
This invention relates in general to offshore drilling equipment and in particular to a tieback system for connecting a subsea well to a platform.
2. Description of the Prior Art
In a typical tieback system, a string of pipe will connect a subsea wellhead housing to a platform. A variety of tieback tools are employed to make the connection between the subsea wellhead and the lower end of the tieback string. In one type, the tieback string is lowered into engagement with the exterior of the wellhead housing. Then a running tool is lowered on drill pipe through the tieback string for making up a sealing connection with an internal casing hanger, typically by rotation of the drill pipe.
Occasions arise in which a tieback is desired to be made internally within a wellhead without using another running tool lowered through the tieback string. For example, when drilling in water several thousand feet deep, an external riser string with a blowout preventer at the surface will connect the subsea wellhead to the platform, which may be a tension leg platform. It may be desired to drill further through the subsea wellhead using an internal drilling riser within the external riser. The internal drilling riser will also have a blowout preventer at the surface. It would be important to be able to make up the tieback string in sealing engagement with the casing hanger within the subsea wellhead without the need for lowering a running tool through the internal drilling riser string.